Certain commercial steel compositions require relatively low amounts of carbon (less than 0.035%), nitrogen (less than 50 ppm), and sulfur (less than 30 ppm). In the past, methods of producing these low carbon and low sulfur steels used a combination of processes in a steelmaking furnace and a degasser. The prior method involved reducing the carbon levels in the steel composition in the steelmaking furnace, such as an electric arc furnace (EAF), making alloy additions during the tapping process to desulfurize and alloy the steel, and then shipping the steel to the degasser such as a vacuum tank degasser (VTD). This processing route was simple and quite straightforward.
To achieve the steel composition requirements of such commercial grades in the past, steel with very low carbon levels, such as less than 0.025%, was tapped at the steelmaking furnace. The dissolved oxygen levels associated with these low carbon amounts were in the order of 1200 ppm to 1400 ppm in the furnace before tapping. Where the degasser was a distance from the furnace, the steel was tapped at approximately 1700° C. to compensate for temperature losses during transportation to the degasser. During the tapping process, the steel was deoxidized with aluminum and ferrosilicon (FeSi). Lime and aluminum dross were also added to create a fluid, deoxidized, desulfurizing slag. By these additions, the desulfurizing reaction was started in the ladle during shipping to the degasser. At the degasser further additions of aluminum, lime, calcium aluminate and dolomitic lime were made to ensure desired sulfur removal during the degassing cycle. While aluminum is used as the primary deoxidant, these steel compositions are commercially considered to be silicon-killed steels.
The prior process had drawbacks, including high refractory wear on the steelmaking furnace. The elevated tapping temperatures and high oxygen content required before tapping the steelmaking furnace had an adverse effect on productivity at the furnaces. The high temperatures and high oxygen conditions enabled high amounts of FeO in the slag at the high temperatures, causing excessive refractory wear on the furnace walls. This led to increased furnace down-times while the furnace refractories were patched with gunite. Also the high FeO content in the slag results in lower efficiency in steelmaking as more iron units are lost in the slag.
The prior process also required the use of low carbon alloys and additives throughout the subsequent processes from the steelmaking furnace to maintain the low carbon level below 0.035% by weight. Low carbon alloying elements, such as low carbon FeMn, were required to provide desired elements without upsetting the final carbon content of the steel. Recently, the price of low carbon ferro-alloys has increased significantly, making this method economically undesirable to produce such low carbon steel. Further, lowering the amount of carbon in the steel composition in the steelmaking furnace required additional decarburization time, which also adversely affected productivity at the steelmaking furnace. Cost was further increased as a result of more silicon and aluminum required to deoxidize the steel composition as a result of the higher oxygen content. There remains a need to decrease production costs of low carbon, low nitrogen, and low sulfur steels.
We have found an alternative method of making a steel with low carbon less than 0.035% by weight that avoids the need for low carbon ferro-alloys, reduces wear on refractories, and increased steelmaking efficiency.
Disclosed is a method of making a steel with low carbon less than 0.035% by weight comprising the steps of:
(a) preparing a heat of molten steel composition in a steelmaking furnace to a tapping temperature as desired for desulfurization at a vacuum tank degasser,
(b) tapping open into a ladle the molten steel composition with an oxygen level between about 600 and 1120 ppm,
(c) providing slag forming compound to the ladle to form a slag cover over the molten steel composition in the ladle,
(d) transporting the molten steel composition in the ladle to a vacuum tank degasser,
(e) decarburizing the molten steel composition at the vacuum tank degasser by drawing a vacuum pressure less than 650 millibars,
(f) after decarburizing, adding one or more deoxidizers to the molten steel composition and deoxidizing the molten steel composition,
(g) after deoxidizing, adding one or more flux compounds to desulfurize the molten steel composition, and
(i) casting the molten steel composition to form a steel with low carbon less than 0.035% by weight.
Before casting, the decarburized and deoxidized molten steel composition may be reoxidized by adding free oxygen to molten steel composition to a level between 20 and 70 ppm and a total oxygen content of at least 70 ppm, and then casting the molten steel composition in a twin roll caster to form a steel strip with low carbon less than 0.035% by weight.
The amount of sulfur in the steel composition during the tapping step may be between about 0.02% and 0.06% by weight. The amount of carbon in the steel composition during the tapping step may be between about 0.02% and 0.05% by weight, and the amount of nitrogen in the steel composition less than about 0.005% by weight. Also, the step of preparing a heat of molten steel composition may be performed in an electric arc furnace.
The open tapping step may be carried out at a temperature between about 1600° C. and 1650° C., or a temperature between about 1650° C. and 1700° C., or a temperature between about 1700° C. and 1750° C.
The decarburizing step may be at a vacuum level of between 1 and 650 millibars, or between 350 and 550 millibars, or at or below 530 millibars.
Prior to decarburizing the step, the method of making a steel with low carbon less than 0.035% by weight may also include the steps of:
(i) stirring the molten steel composition in the ladle at the vacuum tank degasser,
(ii) measuring and recording the amount of carbon in the steel composition, amount of oxygen in the steel composition, and the temperature of the steel composition,
(iii) providing a process model correlating amounts of oxygen and carbon in steel composition with decarburization time to reach a desired amount of carbon in the steel composition; and
(iv) determining by the process model a decarburization time based on the measured amounts of oxygen and carbon in the steel composition.
Alternatively or in addition, prior to decarburizing the step, the method of making a steel with low carbon less than 0.035% by weight may further comprise the steps of:
(i) stirring the steel composition in the ladle at the vacuum tank degasser,
(ii) measuring and recording the amount of carbon in the steel composition, amount of oxygen in the steel composition, and the temperature of the steel composition,
(iii) providing a process model correlating amounts of oxygen and carbon in steel composition with amounts of deoxidizing addition needed to deoxidize the steel composition and
(iv) determining by the process model an amount of deoxidizing addition to the steel composition based on the measured amounts of oxygen and carbon in the steel composition.
The step of adding one or more flux compounds may involve adding one or more compounds selected from a group consisting of lime, aluminum, calcium aluminate, dolomitic lime, and ferrosilicomanganese.
Further, alternatively or in addition, prior to decarburizing the step, the method of making a steel with low carbon less than 0.035% by weight may further comprise the steps of:
(i) stirring the steel composition in the ladle at the vacuum tank degasser,
(ii) measuring and recording the amount of carbon in the steel composition, amount of oxygen in the steel composition, and the temperature of the steel composition;
(iii) providing a process model correlating amounts of oxygen and carbon in steel composition with amounts of flux elements to desulfurize the steel composition, capable of selecting one or more flux elements based on the price of the flux elements; and
(iv) determining by the process model a selection of flux elements and their amounts based on the measured amounts of oxygen and carbon in the steel composition.